The present invention relates to polyamide resin composition pellets for molding, e.g., injection molding, and to a process for producing the same. More particularly, the invention relates to a polyamide resin composition and pellets thereof having excellent workability, e.g., releasability and plasticity, under a wide range of injection molding conditions and having excellent color tone quality.
Polyamide resins, especially polyamide 66 resins, are extensively used as injection-molded articles such as connectors and tying bands because of their excellent mechanical properties. In recent years, injection-molded articles having a complicated shape are required and there also is a desire for the fast-cycle production of these articles under various molding conditions.
For example, some molding techniques employ a relatively low cylinder temperature near the melting point of the polymer, while others employ a cylinder temperature as high as 350xc2x0 C. With respect to mold temperature, some techniques use a mold temperature controller to conduct molding at a mold temperature as low as room temperature or below, while others employ a mold temperature as high as 100xc2x0 C. or above.
Important means for raising productivity in injection molding are to improve the releasability of molded articles from the molds and to shorten the time required for plasticization. The technique generally used for attaining these properties is to add various additives to pellets. Examples of lubricants for use in polyamide resins include higher fatty acids, metal salts of higher fatty acids, esters of higher fatty acids, and higher fatty acid bisamides.
However, use of any one of these lubricants only improves releasability in a specific narrow mold temperature range and plasticity in a specific narrow cylinder temperature range, and it is difficult in injection molding to simultaneously satisfy releasability in a wide mold temperature range of from a low to a high temperature and plasticity in a wide cylinder temperature range of from a low to a high temperature.
JP-A-7-228770 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a technique in which a combination of a higher fatty acid metal salt and a higher fatty acid ester is attached to the surface of pellets. Use of these lubricants in combination is effective in improving releasability in a wider mold temperature range than in the case of using a single lubricant. However, the technique in which lubricants are merely attached to the pellet surface as in the disclosed technique has an unsolved problem that releasability is apt to fluctuate because of the unevenness of the amount of the lubricants attached to the surface and detachment of the attached lubricants from the surface. Furthermore, especially in molding with a high temperature melting, the technique described above causes a problem that a longer time is required for plasticization upon injection molding, resulting in reduced productivity, etc. The reason for this problem may be that in the case where pellets having a higher fatty acid ester attached to the pellet surface are fed to a high-temperature cylinder, the lubricant present on the pellet surface melts to cause the pellets to fuse to one another and thereby inhibit the pellets from smoothly moving in the cylinder.
U.S. Pat. No. 5,929,200 discloses a technique in which various carboxylic acid salts are incorporated into a polyamide melt during a polymerization process for polyamide production. However, salts of lower aliphatic carboxylic acids are almost ineffective in improving the moldability.of polyamides, while salts of higher aliphatic carboxylic acids, when used alone, are insufficient for attaining a wide range of molding conditions, which attainment is included in the objects of the invention.
JP-B-6-19016 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses a technique in which a phosphorous ester, a higher fatty acid metal salt having 10 to 25 carbon atoms, and a higher fatty acid ester are incorporated into a polyamide through kneading with an extruder. Indeed, this composition has improved releasability and further retains impact strength. However, the addition of a nucleating agent such as the phosphorous ester is apt to cause problems that the composition gives a molded article which has a reduced elongation and is brittle and that the viscosity of the polyamide becomes unstable.
JP-A-9-235464 discloses a technique in which an ester of a higher fatty acid with a lower dihydric alcohol is used. However, in the case of using this aliphatic carboxylic acid ester, it is difficult to satisfy releasability under a wide range of mold temperature conditions although releasability in a specific temperature range is improved.
An object of the invention is to provide polyamide resin pellets which have improved releasability and reduced fluctuations thereof, both in a wide mold temperature range, even without adding a nucleating agent, have improved plasticity characteristics in a wide cylinder temperature range, and further have excellent color tone quality, so as to cope with the recent needs of the market for improved moldability required of polyamide resins.
Other objects and effects of the present invention will become apparent from the following description.
As a result of extensive investigations, the present inventor has found that those problems can be eliminated by using a specific combination of lubricants and incorporating a specific one of these lubricants inside pellets.
Specifically, the above-described objects of the present invention have been achieved by providing the following polyamide pellets.
1. Polyamide resin pellets which comprise:
100 parts by weight of a polyamide resin;
from 0.01 to 1.0 part by weight of lubricant A, which comprises at least one ester of a higher aliphatic carboxylic acid with a higher alcohol; and
from 0.01 to 1.0 part by weight of lubricant B, which comprises at least one higher aliphatic carboxylic acid metal salt,
wherein said lubricant A is present inside the pellets.
2. The polyamide resin pellets of claim 1 , wherein said lubricant B is a metal salt of a higher aliphatic carboxylic acid having 26 to 30 carbon atoms.
3. The polyamide resin pellets of claim 2, wherein lubricant B is attached to the surface of the pellets.
4. The polyamide resin pellets of claim 1, wherein said lubricant B comprises at least two higher aliphatic carboxylic acid metal salts, at least one of which is attached to the surface of the pellets.
5. Polyamide resin pellets which comprise:
100 parts by weight of a polyamide resin;
from 0.01 to 1.0 part by weight of lubricant A, which comprises at least one ester of a higher aliphatic carboxylic acid with a higher alcohol;
from 0.01 to 1.0 part by weight of lubricant B, which comprises at least one higher aliphatic carboxylic acid metal salt; and
from 0.01 to 1.0 part by weight of lubricant C, which comprises a higher aliphatic carboxylic acid amide,
wherein said lubricant A is present inside the pellets.
6. The polyamide resin pellets of claim 5, wherein said lubricant B comprises at least two higher aliphatic carboxylic acid metal salts, at least one of which is attached to the surface of the pellets.
7. The polyamide resin pellets of claim 5, wherein said lubricant A is an ester of an aliphatic carboxylic acid having 15 to 21 carbon atoms.
8. The polyamide resin pellets of claim 6, wherein said lubricant A is an ester of an aliphatic carboxylic acid having 15 to 21 carbon atoms.
9. The polyamide resin pellets of claim 5, wherein said lubricant C is a saturated fatty acid amide.
10. The polyamide resin pellets of claim 6, wherein said lubricant C is a saturated fatty acid amide.
11. The polyamide resin pellets of claim 7, wherein said lubricant C is a saturated fatty acid amide.
12. The polyamide resin pellets of claim 6, wherein said lubricant B that is attached to the pellet surface is a metal salt of a higher fatty acid having 15 to 30 carbon atoms.
13. The polyamide resin pellets of claim 8, wherein said lubricant B that is attached to the pellet surface is a metal salt of a higher fatty acid having 15 to 30 carbon atoms.
14. The polyamide resin pellets of claim 13, wherein said lubricant C is a saturated fatty acid amide.
Known polyamide resins can be used as the polyamide resin for use in the invention. Examples thereof include polyamide 66, polyamide 610, polyamide 612, polyamide 46, and polyamide 1212, each obtained by the polycondensation of a diamine with a dicarboxylic acid, and further include polyamide 6 and polyamide 12, each obtained by the ring-opening polymerization of a lactam. Examples thereof furthermore include polyamide copolymers such as polyamide 66/6, polyamide 66/610, polyamide 66/612, polyamide 66/6T (T represents terephthalic acid ingredient), polyamide 66/6I (I represents isophthalic acid ingredient), and polyamide 6T/6I. Blends of these polyamide resins are also included in the examples. These polyamide resins may be produced by a generally employed known method. Although a melt polymerization process is generally used for the polyamide production, it may be conducted either batchwise or continuously.
In producing the polyamide resin composition pellets of the invention, a plurality of the specific lubricants described below are added.
Lubricant A, the first lubricant essential to the invention, is an ester of a higher aliphatic carboxylic acid with a higher alcohol. Examples of the higher aliphatic carboxylic acid include fatty acids having 6 to 30 carbon atoms, such as capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, undecylenic acid, oleic acid, elaidic acid, cetoleic acid, erucic acid, brassidic acid, linoleic acid, linolenic acid, arachidonic acid, and stearolic acid. Examples of the higher alcohol include monohydric aliphatic alcohols having 6 to 30 carbon atoms. Preferred examples thereof include hexyl alcohol, octyl alcohol, lauryl alcohol, stearyl alcohol, behenyl alcohol, and oleyl alcohol. The lower alcohols and lower glycols each having 5 or less carbon atoms are undesirable because they are less effective in improving releasability. Esters of those higher aliphatic carboxylic acids with those higher alcohols are used. Examples of such esters include stearyl stearate, which is the ester of stearic acid with stearyl alcohol, and behenyl behenate, which is the ester of behenic acid with behenyl alcohol. Preferred of such esters are esters of aliphatic carboxylic acids having 15 to 21 carbon atoms. This is because esters of aliphatic carboxylic acids having 15 to 21 carbon atoms, during injection molding, are present in a liquid state on the mold surface and hence show improved release properties. Two or more kinds of lubricants A may be used in combination as needed.
Lubricant B, the second lubricant essential to the invention, is a metal salt of a higher aliphatic carboxylic acid. Examples of the higher aliphatic carboxylic acid include fatty acids having 6 to 30 carbon atoms, such as capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, undecylenic acid, oleic acid, elaidic acid, cetoleic acid, erucic acid, brassidic acid, linoleic acid, linolenic acid, arachidonic acid, and stearolic acid. Examples of the metal in the metal salt include alkali metals such as sodium and potassium, alkaline earth metals such as calcium and magnesium, aluminum, zinc, manganese, and nickel.
Two or more kinds of lubricants B may be used in combination as needed. To enhance the releasability and plasticity, it is preferred that at least one lubricants B is attached to the pellet surface. The carbon number of lubricant B that is attached to the pellet surface is preferably from 15 to 30, and particularly preferably from 26 to 30, for further improving the releasability and for remarkably improving plasticity at a relatively low cylinder temperature around the melting point of the polyamide.
Lubricant A and lubricant B can be incorporated into or added to a polyamide resin by the following methods.
In the invention, it is essential that lubricant A be incorporated inside polyamide resin pellets. Lubricant A (fatty acid ester) generally has a lower melting point than lubricant B (fatty acid salt) Therefore, if pellets having lubricant A attached to the surface thereof are fed to a high-temperature cylinder, this lubricant present on the pellet surface melts to cause the pellets to fuse to one another and thereby inhibit the pellets from smoothly moving in the cylinder. This causes a problem that the plasticization time is prolonged.
There are several methods for enabling lubricant A to be present inside the polyamide resin pellets.
In one method, lubricant A is first heated to or above the melting point thereof and this melt is injected into a molten polymer during a polymerization process for polyamide production, whereby the lubricant can be present inside the resultant pellets. The temperature at which the lubricant is melted is preferably as low as possible from the standpoint of preventing lubricant pyrolysis. A preferred range of the temperature at which lubricant A is melted is from the melting point of the lubricant to the temperature higher than the melting point by 100xc2x0 C., and a more preferred range thereof is from the melting point of the lubricant to the temperature higher than the melting point by 50xc2x0 C. For the injection, a feed pump such as, e.g., a gear pump or plunger pump is used to inject the molten liquid lubricant into the molten polymer in a polymerization line for polyamide production by means of side injection. In this case, it is preferred to dispose a line mixer, e.g., a static mixer, in the piping because the lubricant can be efficiently dispersed into the polyamide polymer by the mixer. In a polyamide production process in which an aqueous salt solution as a starting material is subjected to polymerization, a large amount of water is generally present in the reaction system in the initial stage of polymerization and, hence, addition of the molten lubricant in this stage of polymerization is apt to result in separation of the lubricant from the aqueous salt solution. It is therefore preferred to inject the lubricant in a later stage of the polymerization process. After the addition of the lubricant, the molten polymer is extruded into strands through orifices and the strands are cut into pellets. Thus, polyamide resin pellets containing the lubricant can be obtained.
Another method comprises mixing the lubricant with pellets of a polyamide resin for which polymerization has been completed and then melt-kneading the resultant mixture again with an extruder to obtain pellets containing the lubricant.
From the standpoints of the precision of lubricant addition, color tone of pellets, and productivity, the method in which lubricant addition is conducted during polymerization for polyamide production is preferred.
Lubricant B as the second lubricant essential to the invention may be attached to the surface of the polyamide resin pellets containing lubricant A, or may be added during polymerization for polyamide production like lubricant A.
Methods which can be used for attaching lubricant B to the surface of pellets include a method in which the lubricant in a powder form is attached as it is by batch blending with a cone blender or continuous blending with a ribbon blender and a method in which the pellet surface is coated with the lubricant with a Henschel mixer or the like. The former method may be conducted in such a manner that polyamide pellets are first introduced into the blender and then sprinkled with a spreading agent, e.g., polyethylene glycol or a polyoxyethylene-sorbitan/fatty acid ester, as an addition aid, before lubricant B is added and mixed therewith. This method is preferred in that lubricant B can be satisfactorily attached to the pellet surface. By attaching lubricant B to the pellet surface, plasticity is especially improved significantly in injection molding employing a relatively low cylinder temperature near the melting point of the polyamide.
On the other hand, the method in which lubricant B is added during polymerization for polyamide production may be conducted in such a manner that lubricant B is first heated, like lubricant A, to obtain a melt and this melt is added to the polymerization system with a pump at the time when the water content in the system has decreased to 10% or lower.
In the case where both lubricant A and lubricant B are injected during a polymerization process, the two lubricants may be separately melted and added. Alternatively, the two lubricants may be mixed with each other, subsequently melted, and then added as a mixture. The method in which the lubricants are mixed before being melted and added is preferred because it necessitates simpler facilities.
The addition amount of lubricant A and that of lubricant B each is from 0.01 to 1.0 part by weight per 100 parts by weight of the polyamide resin. Addition amounts thereof smaller than 0.01 part by weight result in an insufficient releasing effect, while addition amounts thereof larger than 1.0 part by weight cause problems, for example, that the viscosity of the composition upon molding is deteriorated and that the molded article is apt to have silver streaks. The preferred range of the addition amount of each lubricant is from 0.03 to 0.5 parts by weight.
The following has been found. In the case where the method comprising mixing lubricant A with lubricant B beforehand, heating the mixture, and then adding the resultant melt during a polymerization process for polyamide production is used, the lubricants produce an effect of lowering melting points when used as a three-component lubricant system comprising the combination of the lubricants A and B and further a specific higher aliphatic carboxylic acid amide (lubricant C) as the third lubricant, to thereby enable melting at lower temperatures. Consequently, use of this lubricant system leads to a reduction in the cost of heating energy for melting and, hence, it is preferred to use lubricants A and B in combination with lubricant C. Furthermore, it has surprisingly been found that the addition of lubricant C is effective in giving molded articles of excellent quality having improved transparency.
Lubricant C is an amide of a fatty acid having 6 to 30 carbon atoms. Examples thereof include the monoamides and bisamides of capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, undecylenic acid, oleic acid, elaidic acid, cetoleic acid, erucic acid, brassidic acid, linoleic acid, linolenic acid, arachidonic acid, and stearolic acid. Preferred examples thereof include saturated fatty acid monoamides such as stearamide and behenamide, unsaturated fatty acid monoamides such as erucamide and oleamide, and fatty acid bisamides such as ethylenebis (stearamide).
The addition amount of lubricant C is from 0.01 to 1.0 part by weight per 100 parts by weight of the polyamide resin. Addition amounts thereof smaller than 0.01 part by weight result in an insufficient effect in lowering the temperature for melting, while addition amounts thereof larger than 1.0 part by weight cause problems, for example, that the viscosity of the composition upon molding is deteriorated and that the molded article is apt to have silver streaks. The preferred range of the addition amount thereof is from 0.03 to 0.5 parts by weight.
The addition of the lubricants described above to a polyamide may be conducted in such a manner that a mixture of these lubricants is prepared beforehand and melted by heating and this melt is added to a polyamide melt in a polymerization process with a feed pump such as those shown above. This addition method is advantageous in that the lubricants can be highly evenly mixed together and the addition amounts thereof can be controlled exceedingly precisely. Because of this, resin pellets having reduced fluctuations of lubricant content and stable moldability can be produced as compared with the case where lubricants are merely attached to the surface of pellets or where pellets are kneaded together with lubricants by means of an extruder.
The polyamide resin composition pellets of the invention may contain known substances according to need. Examples of such optional ingredients include polymerization catalysts such as phosphorus compounds, e.g., hypophosphorous acid salts, phosphorous acid salts, and phosphoric acid salts, nucleating agents such as phosphorous esters and phenylphosphonic acid salts, polymerization degree regulators such as acetic acid, heat stabilizers such as copper acetate, copper iodide, potassium iodide, and potassium bromide, fillers such as talc, kaolin, and wollastonite, reinforcements such as glass fibers and carbon fibers, and light stabilizers such as ultraviolet absorbers and HALS.